1. Field of the Invention
The present invention relates generally to a method for improving the resolution of nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to a method for compensating for the effects of commonly used processing methods for eliminating any ringing, such as magnetoacoustic ringing, and DC offset, during a nuclear magnetic resonance measurement.
2. Background of the Art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
The NMR instrument also typically includes an antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the field applied by the magnet. This RF pulse, typically called a 90° pulse, has a duration and amplitude predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicularly both to the orthogonal magnetic field induced by the RF pulse and to the magnetic field applied by the magnet. After the 90° pulse ends, the nuclear magnetic moments of the hydrogen nuclei gradually “relax” or return to their original alignment with the magnet's field. The amount of time taken for this relaxation, referred to as T1, is related to petrophysical properties of interest of the earth formation. After the 90° pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. The spin axes of the hydrogen nuclei gradually “dephase” because of inhomogeneities in the magnet's field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. Typically, a series of 180° refocusing pulses are applied to bring the spins back into focus. Each refocusing pulse produces an echo, and from analysis of the echo train, properties of the earth formation can be estimated
One problem with analysis of NMR measurements is that the signal detected by the antenna includes a parasitic, spurious ringing that interferes with the measurement of spin-echoes. One approach to reduce the effects of ringing has been to design the hardware to minimize the interaction between the electromagnetic fields and the materials in the device. For example U.S. Pat. No. 5,712,566 issued to Taicher et al. discloses a device in which the permanent magnet composed of a hard, ferrite magnet material that is formed into an annular cylinder having a circular hole parallel to the longitudinal axis of the apparatus. One or more receiver coils are arranged about the exterior surface of the magnet. An RF transmitting coil is located in the magnet hole where the static magnetic field is zero. The transmitting coil windings are formed around a soft ferrite rod. Thus, magnetoacoustic coil ringing is reduced by the configuration of the transmitting coil. Magnetorestrictive ringing of the magnet is reduced because the radial dependence of the RF field strength is relatively small due to use of the longitudinal dipole antenna with the ferrite rod. Further, magnetorestrictive ringing is reduced because the receiver coil substantially removes coupling of the receiver coil with parasitic magnetic flux due to the inverse effect of magnetostriction.
Another commonly used approach to reduce the effect of ringing is to use a so-called phase-alternated-pulse (PAP) sequence. Such a sequence is often implemented asRFA±x−τ−n·(RFBy−τ−echo−τ)−TW   (1)where RFA±x is an A pulse, usually 90° tipping pulse and RFB is a B pulse, usually a 180° refocusing pulse. The ± phase of RFA is applied alternately in order to identify and eliminate systematic noises, such as ringing and DC offset through subsequent processing. By subtracting the echoes in the −sequence from the pulses in the adjoining +sequence, the ringing due to the 180° is suppressed.
PCT publication WO 98/43064 of Prammer addresses the problem of ringing caused by the excitation pulse. A dual frequency acquisition is carried out with phase alternation, the separation between the two frequencies being one fourth of the reciprocal of the delay time in acquisition between the excitation pulse and the first refocusing pulse. Averaging of the two measurements then attenuates the effect of the ringing due to the excitation and refocusing pulses.
A drawback to the averaging of phase alternated data sequence is the requirement to combine two pulse sequence cycles. Measurements made by an NMR logging tool in this manner are therefore subjected to degradation in the vertical resolution due to the logging speed, wait time between each pulse sequence, and the data acquisition time. In addition, the logging tool moves along the longitudinal axis of the borehole between each of the measurements. The problem with logging speed is exacerbated in multifrequency NMR measurements. A pulse sequence for an eight frequency logging operation may be denoted byCPMG(f1, TE1, RFA+, n1)−t1−CPMG(f2, TE2, RFA+, n1)−t2− CPMG(f8, TE8, RFA+, n8)−t8−CPMG(f1, TEi, RFA−, n1)−t1−CPMG(f2, TE2, RFA−, n2)−t2 . . . CPMG(f8, TE8, RFA−, n8)−t8   (2)where fi, TEi and ni are the frequency, interecho time and number of echoes for the i-th CPMG echo train. The CPMG pairs that only differ in the RFA phases are 8 sequences apart. Unless the logging speed is slowed down significantly, the two sensed volumes will be spatially separate and distinct, and the resolution of the tool is impaired.
It would be desirable to have a method of improving the resolution of NMR data that has been averaged over a depth interval. The present invention satisfies this need.